A thin-film transistor (TFT) is known as a thin-film semiconductor device fabricated on a glass substrate. TFTs formed on such a glass substrate are disposed in a pixel driver portion and also in a peripheral circuit for a liquid crystal display and are used to display images with high information content. Furthermore, these TFTs are employed in image sensors and in other integrated circuits.
Where a glass substrate is used, the following advantages can be derived:
(1) Since it is optically transparent to visible light, the glass substrate can be easily utilized in a device such as a liquid crystal display through which light is transmitted.
(2) It is inexpensive. However, the upper limit of the thermal treatment temperature is restricted by the heatproofness, i.e., the maximum usable temperature, of the glass substrate.
Corning 7059 glass is generally used as a glass substrate taking account of the problem of impurity release from the glass substrate, price problem, and other problems. The transition point of this glass is 628.degree. C. and the strain point is 593.degree. C. Other known practical industrial glass materials having strain points of 550-650.degree. C. are listed in Table 1 below.
TABLE 1 7059D(CGW) 7059F(CGW) 1733(CGW) LE30(HOYA) TRC5(OHARA) E-8(OHARA) N-0(NEG) OA2(NEG) strain 593 593 640 625 643 625 point (.degree. C.) thermal 50.1 50.1 36.5 38.0 52.0 37.0 -7.0 38.0 expansion coefficient (.times.10.sup.-7) transmission 89.5 89.5 91.9 90.0 N.A. 91.0 N.A. 90.0 (%) (400 nm) (400 nm) (400 nm) (450 nm) (450 nm) (450 nm) composition SiO.sub.2 49 49 57 60 59 60 Al.sub.2 O.sub.3 10 10 16 15 15 15 B.sub.2 O.sub.3 15 15 11 6 7 6 R.sub.2 O 0.1 2 1 2 AN1(AGC) AN2(AGC) NA35(HOYA) NA45(HOYA) strain 625 616 650 610 point (.degree. C.) thermal 44.0 47.0 39.0 48.0 expansion coefficient (.times.10.sup.-7) transmission 90.0 89.8 N.A. N.A. (%) (500 nm) (500 nm) composition SiO.sub.2 56 53 51 Al.sub.2 O.sub.3 15 11 11 B.sub.2 O.sub.3 2 12 13 R.sub.2 O 0.1 0.1 0.1
Where an amorphous silicon film formed on a glass substrate by CVD is crystallized by heating, a high temperature, e.g., above 600.degree. C., is generally needed. Therefore, where a Corning 7059 glass substrate is used, the substrate is shrunk by the heating.
An active-matrix liquid crystal display is known as a device utilizing TFTs formed on a glass substrate. To fabricate this liquid crystal display, it is necessary to form tens of thousands to several millions of TFTs on the glass substrate in rows and columns. To manufacture the TFTs, processes using numerous masks are necessitated. Consequently, shrinkage of the substrate is a great impediment to the manufacturing process.
Especially, where it is necessary to make a mask alignment before thermal treatment, substrate shrinkage caused by the thermal treatment is a problem.
In a process for heat-treating substrates, it is common practice to place these plural substrates in vertical posture within a heating furnace, taking account of the processing speed. Where the substrates are heated above their strain point, warpage of the substrates is conspicuous.
Where TFTs are formed on a glass substrate, if the used TFTs permit flow of a large electrical current, then generation of heat accompanying the operation is a problem. This problem associated with the heat generation arises from the difference in coefficient of thermal expansion between silicon and the glass substrate. That is, the coefficient of thermal expansion of silicon, i.e., a single crystal of silicon, is 148 W m.sup.-1 K.sup.-1 (300 K), while the coefficient of thermal expansion of the glass substrate, i.e., quartz glass, is 1.38 W m.sup.-1 K.sup.-1 (300 K). Since the coefficient of thermal expansion of the glass substrate is much lower than that of silicon in this way, during operation of TFTs, heat generated by the TFTs cannot escape. Hence, malfunction and thermal destruction due to the heat generation present problems.
Especially, these problems become conspicuous where crystalline silicon is used. In particular, TFTs using an amorphous silicon film treat weak electrical current and so the problem of heat generation is not so serious. On the other hand, TFTs using a crystalline silicon film permit flow of large electrical current. Therefore, generation of heat poses a great problem.